Significant research has been done on the clotting of blood, and there are many excellent texts available on this subject. Of the many factors that stimulate blood clotting, the presence of a foreign substance in the bloodstream is one that is well recognized. Metal objects in the bloodstream, such as stents, can stimulate clotting because of their chemical/physical makeup and/or because of the physical disruption of blood flow. It is often difficult to determine which of these two factors affects clotting the most. Recent clinical work associated with the implantation of metal stents in patients treated with a variety of antithrombogenic agents, such as coumadin and aspirin, prior to implantation of the stent and for a period of time post implantation, suggests that unwanted clotting is moderately easy to control during the critical 2-4 week period following stent implantation. Still, clotting during this time period is of great concern to physicians and manufacturers alike. Therefore, the market appears to be driven toward lower profile and less thrombogenic products and/or implantation techniques that minimize the impact of these devices on the normal physiology of the blood.
Other studies associated with stent implantation have shown that the placement of metallic stent material against the arterial wall can stimulate the growth of the inner lining of the arterial wall to form a neointima over the surface of the stent. Neointimal growth over the metal stent surface serves two functions. First, it serves to smooth the path for the blood flow, thereby reducing the amount of turbulence. Secondly, it can help isolate the metal from contact with the blood reducing the chemical interaction of the stent and the blood. The combined clot-reducing effects of this phenomenon has helped to reduce physician concern regarding the thrombogenic potential of properly placed stents after the neointima has formed.
Knowledge of the above clotting factors has caused some manufacturers of balloon expandable stents (BES) to develop stents with reduced strut thickness. However, if the stent strut is made too thin, it will lose its effectiveness as a support scaffold for the vessel. Therefore, most BES stents still have a relatively thick strut cross sectional area. To minimize the impact of the large cross sectional area it is common practice to implant the stent at relatively high pressures. This solves two problems for the user. It helps to bury the stent within the vessel wall thus reducing or eliminating much of the strut induced turbulence. It also helps to overcome a phenomenon called "rebound" or "recoil". Rebound is an elastic characteristic of metal that has been deformed. The metal wants to return to its undeformed state. In most BES devices, this causes the stent to contract up to twenty percent from its expanded diameter. Unfortunately, the practice of implanting stents deeply into the vessel wall frequently causes many tears or dissections of the vessel wall. Some of these tears can lead to flaps that protrude into the blood stream. In addition, the small surface area of BES struts when expanded (the stent covers less than 20% of the vessel wall) concentrates the expansive force of the stent on a relatively small area of the medial layer of the vessel. This concentrated stress can lead to inflammation of the vessel at the site of the stent struts. The tear or flap can cause turbulence in the flow of the blood and that combined with inflammation of the vessel wall can add to other complex factors resulting in restenosis of the vessel. This is the very problem the user is trying to prevent by placement of the stent.
As explained above, most currently used peripheral stents have many large open spaces between the stent struts. These open spaces provide a conduit for the blood to stimulate intimal hyperplasia in the damaged areas, with growth forming right through the open matrix of the stent. In order to help isolate the damaged area from the flow of blood, several physicians have developed a technique of placing a suitable graft material on the outside of the stent prior to implantation, thus creating a stent-graft. Typically, the graft material is an existing tubular form of Dacron or ePTFE of a size and thickness commonly used by those skilled in the art of vascular repair (0.010 inch wall thickness with a porosity of 220 cc/cm.sup.2/min or less and a length and diameter determined by the size of lesion to be repaired). While this technique has increased the user's ability to isolate the arterial wall from the flow of blood, it has also reduced or eliminated the ability for the neointima to grow over the graft and stent material. In addition, it further increases the profile of the stent by reducing the physician's ability to imbed the stent into the arterial wall. This technique may actually aggravate turbulence-induced thrombogenicity problems associated with the use of graft material on the outside of a stent, especially in the smaller vessels where blood flow rates are reduced.
Thus, there remains a need for a stent design with sufficient radial strength and structure for permitting neointimal tissue ingrowth, as well as the capability to treat vascular sites which may be of small diameter and/or extend along curved portions of the vessel. In addition, there remains a need for grafts which can be readily implanted, which do not place concentrated stress on the vessel wall, are flexible to track curved portions of the vessel, and which provide sufficient support to maintain patency.